This invention relates to a method for producing a semi-conductor body useful in a jet engine igniter of the high energy type. In service such an igniter is fired by a capacitor discharge ignition system. The semi-conductor body is incorporated in the high energy igniter so that a portion of a surface thereof is adjacent a spark gap between a center electrode and a ground electrode. It has been found that a semi-conductor, so positioned, reduces the voltage required to cause a spark discharge, by comparison with an igniter where there is an alumina insulator in this position. Although the mechanism by which a semi-conducting body operates to reduce the voltage requirement is not fully understood, two theories have been proposed. These theories are stated below, but the statement should in no way be construed as a limitation on the scope of this invention.
One theory suggests that when a voltage is applied to the center electrode, there is a limited flow of current along the semi-conductor surface. This current flow causes ionization of gas in the spark gap. The ionization enables a spark discharge to occur at a lower voltage than would be required without the ionization.
Another theory suggests that because a small space of about 0.0005 inch exists between the center electrode and the semi-conducting body, electrical charges of opposite polarity build up on the surfaces of the center electrode and of the semi-conductor, as in the polarization of opposing faces of a capacitor. Ionization of gas in this small space or microgap within the igniter gap enables an initial spark discharge at a low applied voltage. This partial discharge is believed to cause a cascade ionization and discharge across the main gap.
In either case, discharge of the previously charged capacitor occurs when there is a spark between the ground and center electrodes. The large size of the capacitor is responsible for the high energy nature of the spark.
An extension of the second theory proposes that the porosity of the semi-conductor surface assists the cascade process by providing a series of microgaps between conducting silicon carbide grains which may become charged, ionized and discharged in rapid succession. The presence of a non-conducting phase such as glass or alumina serves not only to bond the conducting grains of silicon carbide, but also to prevent a direct short circuit. Controlled spacing and contact of the silicon carbide grains is obtained by means of the porosity and the glass or alumina as well as the grain size and amount of silicon carbide.
Various electrically semi-conducting ceramic bodies have heretofore been suggested and used in igniters for low voltage ignition systems. The prior art has emphasized, insofar as semi-conductors containing silicon carbide are concerned, such semi-conductors having a crystalline bonding phase. For example, U.S. Pat. No. 3,558,959 discloses alumina and silicon carbide semi-conductors having a crystalline bond produced by hot pressing the alumina and silicon carbide. U.S. Pat. Nos. 3,376,367 and 3,573,231 disclose the production of crystalline bonded semi-conductors from silicon carbide and aluminum silicate or the like by forming an article of the desired shape, firing in air to achieve a controlled oxidation of silicon carbide to silica, embedding the article in a mass of silicon carbide particles, and firing the article while so embedded. Alternatively, the aluminum silicate can be a part of the batch from which the original shape is formed. In either case the bonding phase is a crystalline aluminum silicate or the like.
The patents described above disclose high energy igniters containing silicon carbide and a bonding matrix which is essentially a crystalline phase. While these crystalline bonded igniters perform well under service conditions, several advantages are obtained with a bonding matrix that is a glass. With such a matrix a lower firing temperature can be used and the porosity of the fired semi-conductor can be controlled more effectively to obtain an increased open porosity, and decreased spark erosion rate.
A silicon carbide semi-conductor having a glassy bonding phase can be produced from a shape of a particular composition by a two-step firing procedure. The shape is first fired in air to reduce the size of the silicon carbide and to introduce SiO.sub.2, and is then fired in an inert atmosphere. An igniter including a silicon carbide semi-conductor having a glassy bonding phase has been produced commercially since 1973. The semi-conductor composition, after firing, was 30.0 percent SiO.sub.2, 9.0 percent Al.sub.2 O.sub.3, 7.2 percent CaO, 1.8 percent MgO and 52 percent SiC. The glassy bonding phase of this semi-conductor contained 62.5 percent SiO.sub.2. 18.8 percent Al.sub.2 O.sub.3 and 18.8 percent CaO plus MgO. The production of semi-conductors by the two-step firing procedure is described in detail in the parent co-pending application Ser. No. 321,563, filed Jan. 8, 1973, now abandoned; it can be used to produce semi-conductors where the composition of the glassy bonding phase, after firing, is from 48.8 to 71.5 percent SiO.sub.2 ; from 13.6 to 32.7 percent Al.sub.2 O.sub.3 ; and from 9.1 to 30.1 percent CaO and MgO.
An improved semi-conductor having a substantially reduced erosion rate when sparked at a pressure of 400 psi. has now been discovered. The improved semi-conductor has an apparent porosity of 20 to 40 and consists essentially of silicon carbide particles dispersed in a bonding matrix; it can be produced by a one-step firing in an inert atmosphere, by the two-step procedure discussed in the preceding paragraph, and, perhaps, by hot pressing. The method, in any case, involves careful control of the apparent porosity of the semi-conductor body; the composition of the bonding matrix appears to be of only minor importance.